Method and system for distinguishing spatial and thermal defects on perpendicular media

ABSTRACT

Disclosed are a method and system for distinguishing spatial and thermal defects on perpendicular media. The magnetic domains of the perpendicular media are oriented to have a first polarity, scanned using a read head, oriented to have a second polarity and scanned again. The signals from the read head are combined to produce output signals having improved signal to noise ratios from which the locations of spatial and thermal defects can be identified and distinguished.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Pat. No. 7,532,422 (U.S. Ser.No. 11/552,930 filed Oct. 25, 2006), titled “Method and System forDistinguishing Spatial and Thermal Defects on Perpendicular Media”, thecontents of which is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of testing magnetic recordingmedia, and more particularly to a method and system for identifying anddistinguishing spatial and thermal defects on the surface ofperpendicular media.

BACKGROUND OF THE INVENTION

Hard disks drives have become ubiquitous for high volume, non-volatilestorage of electronic data. While principally used as data storagedevices for computing systems, hard drives have found additional uses,including, for example, in video and audio recording systems, and insmall, highly portable music playback systems. As with many types ofelectronic devices, very substantial efforts have been made over recentyears to increase the performance of hard disk drives. These effortshave primarily been directed to increasing hard disk storage capacity,reliability and robustness, while reducing cost, size, and data access(read/write) times.

In hard disk drives, data is stored on a spinning hard disk or platter,using a recording head, in digital form, as a series of binary bits,each of which is stored at a precise, known, physical location on asurface of the disk. Typically, modern hard disk drives comprisemultiple, coaxial, stacked platters, each of which comprises an aluminumor other substrate having a magnetic film deposited on both the upperand lower surfaces of the platter. As is well known in the art, data isstored by the polarization of the magnetic domains in small,well-defined areas of the magnetic film on the platter. The magneticdomains are oriented using a disk drive write head comprising a coilused to transmit a precise electromagnetic signal to orient themagnetization of a domain on the surface of the disk immediatelyadjacent to the head. In this manner, the magnetic field at the surfaceof the disk at a given location is made to represent either a logical 1or 0, corresponding to the desired binary bit value, and can thereafterbe read back (or changed) using the read/write head. In practical terms,as is well known, the data is actually stored in the form of magnetictransitions from one domain to the next.

It can be appreciated that the amount of data that can be stored on ahard disk drive is a function of both the overall available area on thedisk surface and the area required to store each bit (including the areanecessary to separate adjacent bit storage locations). For practicalreasons, the size of the disks has actually been decreasing.Accordingly, in order to increase storage capacity, great attention hasbeen paid to reducing the already very small area on the surface of thedisk necessary to store data bits. Important factors in this effortinclude reducing the separation distance between the read/write head andthe disk surface (the “flying height”), improving the uniformity of themagnetic film, and reducing the size of the domains so that very smallareas on the usable surface of a disk can be reliably used for datastorage. However, as the areal extent of the magnetic surface used forstorage of a data bit is decreased, any small defects or imperfectionsin the area take on greater significance.

In traditional prior art hard disk drives the magnetic domains 10 arehorizontally or “longitudinally” aligned on the surface of the magneticfilm as depicted in FIG. 1A. Reversing the magnetization of a domainrelative to the adjacent domains, causes a magnetic transition 20 whichis detectable when a read/write head passes over the transition area anddetects a variation in the magnetic flux above the surface of themagnetic film. However, there are practical limits to the size ofhorizontal magnetic domains. Specifically, after a limit is reached,smaller magnetic domains are inherently unstable due to thermalfluctuations.

New generations of hard disk drives use vertically polarized magneticdomains 30 to reduce the amount of space needed to store data, as shownin FIG. 1B. Again, magnetic transitions 40 between adjacent domains canbe created using a write head and, thereafter, detected using a readhead. One estimate is that vertical magnetic polarization, or“perpendicular” data storage, can increase the storage capacity of adisk ten-fold. However, vertical drives use thicker magnetic films andrequire a “soft” magnetically permeable underlayer 50, which canincrease the manufacturing difficulty of achieving a highly uniform,planar surface.

Normally, the disk surface may be viewed as comprising a plurality ofcontiguous annular regions or “tracks” that are used for data storage.Track widths of vertical hard disks are of the order of 100 nanometers,and track density is of the order of 2,400,000 transitions per inch.

As can be understood from FIG. 1B, when all of the magnetic domains of avertical disk are aligned in the same direction, i.e., when there are notransitions, the entire surface has a single magnetic polarity and themagnetic field adjacent to the surface of the disk is substantiallyuniform. In contrast, when there are no transitions in a horizontaldisk, the magnetic field varies with location.

After manufacture, the platters of a hard disk drive need to be testedfor defects and to ensure that they meet specifications. Testing istypically performed on unformatted disk platters prior to final diskdrive assembly. For the reasons discussed above, the specifications arebecoming more stringent as smaller disk areas are used for data storage.Small scratches, pits and other defects in the surface of the magneticfilm are particularly critical and the existence of any such defectsneeds to be identified. By identifying the location of spatial defects,data loss is avoided by marking the area as defective prior to use, orby discarding the disk entirely if it is found to have too many defects.It is noted that vertical domains can be smaller than optical detectionlimits, such that optical inspection of the disk surface cannot be usedto identify surface irregularities that impact device performance.

In addition to spatial defects such as scratches or other irregularitiesin the disk surface, hard disks are also subject to “thermal” defectsthat may occur along with or separately from the spatial defects. Thistype of defect is essentially a small bump or protrusion on the surfaceof the platter, where the height of the bump is such that the read headmakes contact with the bump, but is able to continue scanning thesurface of the disk (i.e., the bump is not so large that the read headstops functioning). When the read head encounters the bump, the highspeed impact causes the read head to increase in temperature (hence thename “thermal” defect). Repeated impacts lead to wear on the read headand can eventually cause the head to “crash” into the surface of thehard disk. As such, thermal defects on a hard disk pose an even greaterproblem than spatial ones. While spatial defects may limit the amount ofdisk space available to store data, thermal defects may cause the harddisk to crash, such that data on the disk may or may not be recoverable.For this reason, a disk having predominantly spatial defects and fewthermal ones will be more usable than a disk having the same totalnumber of defects, but where a substantial number of the defects arethermal. Accordingly, there is a need for a system and method toidentify spatial and thermal defects on a vertical hard disk platter andto distinguish each type of defect from the other.

A common testing technique currently in use is referred to as the“missing pulse test.” The missing pulse test involves writing asinusoidal waveform to the surface of the disk using a write head, andthen reading back the recorded signal using a read head. Because a sinewave has two transitions per cycle, the read back frequency is twice thewrite frequency. Discrepancies, referred to as “dropouts”, between whatis written and what is read are used to identify disk errors. As domainsize has decreased, it has become necessary to use higher frequency toproperly analyze the surface of a disk. Currently, write frequencies ashigh as 200 MHz (and corresponding read frequencies of 400 MHz) may beused.

SUMMARY OF THE INVENTION

As part of making their invention, the inventors have found that defectson perpendicular media can be identified by scanning over the media witha read head when the media is aligned in polarity. Further, when themedia is scanned multiple times, with the polarity reversed for eachscan, the resulting signals can be combined not only to identify thelocation of defects, but to distinguish spatial defects from thermalones.

Accordingly, the present invention is directed to a method and systemfor testing hard disk platters having vertically oriented magneticdomains to identify and distinguish spatial and thermal defects. In oneembodiment, the invention is directed to a method of inspecting anunformatted hard disk platter having vertically oriented magneticdomains for spatial and thermal defects, comprising the steps oforienting the magnetic domains in the same vertical direction andscanning the surface of the disk using a read head to identifyperturbations in the magnetic field intensity, orienting the magneticdomains in the opposite vertical direction and scanning the surfaceagain, then combining the resulting signals from the read head toidentify the locations of defects and to differentiate between spatialand thermal defects. The step of orienting the magnetic domains can beaccomplished by performing a DC erase of the disk surface, such as in abulk erase operation. Alternatively, the step of performing a DC erasemay comprise using a write signal from a write head to sequentiallyorient domains. Preferably, the step of scanning the surface of the diskcomprises sampling the signal picked up by the read head at a highfrequency.

In another aspect, the present invention is directed to a system forquality testing hard disks having vertically oriented magnetic domainsto distinguish spatial and thermal defects, comprising a write head fororienting the magnetic domains of the hard disk in the same direction, aread head comprising a magnetic sensor for detecting the magnetic fieldat the surface of the disk adjacent to the read head, a transportmechanism for providing relative motion between the surface of the diskand the read head such that the read head is able to accesssubstantially the entire useable surface of the disk, and a signalanalyzer for analyzing the output signal from the read head, wherein thesignal analyzer comprises a signal processor for adding and subtractingthe signals to produce output signals having improved signal to noiseratios.

These and other features of the present invention will become apparentto those skilled in the art from the following detailed description ofthe invention, the accompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross-sectional depiction of a portion of thesurface of a hard disk having a horizontal magnetic domain orientation.

FIG. 1B is a schematic cross-sectional depiction of a portion of thesurface of a hard disk having a vertical magnetic domain orientation.

FIG. 2A is a schematic cross-sectional depiction of a portion of thesurface of a vertically oriented hard disk platter having a spatialsurface irregularity.

FIG. 2B shows the output signal from a read head as it passes over thedefect shown in FIG. 2A.

FIG. 3 illustrates the sensor resistance of the read head thatcorresponds to the magnetic field of the hard disk.

FIG. 4 shows the effect of a change in temperature on the sensorresistance.

FIG. 5 shows the sensor response between scanning defective anddefect-free areas of the disk for a first polarity.

FIG. 6 shows the sensor response between scanning defective anddefect-free areas of the disk for a second polarity.

FIG. 7 shows sensor head output signals for first and second polaritiesof the present invention along with an output signal of the conventionalmissing pulse technique.

FIGS. 8A-8D show the post-processing signals for spatial and thermaldefects according to the present invention in comparison with those ofthe conventional missing pulse technique.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, numerous specific details are set forth toprovide a more thorough description of the specific embodiments of theinvention. It is apparent, however, that the invention may be practicedwithout all the specific features discussed below. In other instances,well known features have not been described in detail so as not toobscure the invention.

The preferred embodiment of the present invention is directed to amethod and system for testing the surface of a hard disk platter havingvertical magnetic domains to distinguish spatial and thermal defects.The testing contemplated by the present invention is performed after thedisk platters are made, but prior to final assembly and formatting ofthe hard drive. The basic hardware used for testing hard disk plattersare known and need not be described in detail. Generally, such devicesinclude a mechanism for rotating the disk, comprising a spindle and oneor more radially translatable heads for reading and writing to the disk.Typically, radial head translation uses a carriage assembly. Thecombination of the disk rotation mechanism and the head translationmechanism constitute a transport mechanism by which a head can accessthe entire usable surface of the disk.

Commercially available head units frequently combine both read and writefunctionality in a single unit, referred to as a read/write head, thatis mounted on a carriage assembly. Embodiments of the present inventioncan be implemented using either combined read/write heads, or separateread and write heads, each of which is mounted on a separate carriage.The latter approach is preferred for the testing method and system ofthe present invention because separate heads provide greater flexibilityand control.

The techniques for manufacturing hard disk platters with verticalmagnetic domains are known in the art and are not considered to be partof the present invention. Accordingly, the manufacturing techniques willnot be described, except to note that the magnetic recording layer isrelatively thicker than in a disk having horizontally oriented magneticdomains, and that vertical disks require the use of a relatively thickunderlayer of highly permeable magnetic material to act as a return pathfor the magnetic flux lines. The greater thickness of these layers addsmanufacturing complexity and increases the difficulty of obtaininghighly uniform films, thereby increasing the likelihood of small surfaceirregularities.

According to the present invention, the vertical magnetic domains on adisk to be tested are all oriented in the same direction. Aftermanufacture, the magnetic domains on the disk platters are randomlyoriented. Accordingly, action is required to orient all of the domainsin the same direction. Preferably, this is a achieved by performing a DCerase of the disk, in a known manner. In one technique, a bulk DC eraseis performed using a degausser. In an alternate technique, the verticalmagnetic domains are oriented in the same direction using a write head.In this alternate technique, it is not necessary that all of the domainson the disk be oriented prior to further testing. Instead, for example,testing can proceed on a track-by-track basis, i.e., the domains in atrack are first oriented in the same direction using a write head, andthen the properly oriented track is tested using a read head. Asdescribed above, when all of the domains in a vertically oriented diskare aligned, the magnetic field adjacent to the surface is, absent anydefects, substantially uniform.

According to the present invention, spatial and thermal defects onperpendicular media are distinguished by scanning the verticallyoriented magnetic domains having a uniform first polarity, scanning thedomains having a uniform second polarity and combining the resultingsignals. FIG. 2A schematically depicts a vertical disk surface 60,having vertically oriented magnetic domains 30, with a non-planar region65 representing a “spatial” defect. Non-planar region 65 may be a pit, ascratch, or other surface defect created as an artifact of themanufacturing process or subsequent disk handling. A read head 70 movesrelative to the disk surface 60. As read head 70 passes over spatialdefect 65, the spacing between read head 70 and the surface of defect 65increases and, as a consequence, the magnitude of the magnetic fluxdetected by head 70 decreases.

The read head 70 is preferably a giant magnetoresistive (GMR) sensor. AGMR sensor comprises a material whose resistance varies in response tochange in a magnetic field, thus producing an output signal thatcorresponds to variations in the magnetic field along the surface of theperpendicular media. FIG. 2B shows the output signal 75 from read head70 as it passes over defect 65 and the adjacent planar regions. Thechange in resistance is used, in accordance with the present invention,to identify film defects.

FIG. 3 depicts the relationship between the magnetic field and sensorresistance of a GMR sensor. As is well known, a GMR sensor requires theuse of a bias field. A bias field may be applied to the sensor using asmall permanent magnet (not shown) or by other known means. Applicationof the bias field 80 positions the resistance of the sensor in theworking range of curve 90, i.e., the portion of the curve that isessentially linear.

A GMR sensor is also sensitive to changes in temperature, increasingresistance with increases in temperature. As shown in FIG. 4, when thetemperature of the sensor is increased, the curve 100 is translatedupward from the original curve 90. Thus, for any given magnetic field, ahigher sensor resistance will be measured when the temperature has beenincreased. These two sensor responses, as shown in FIGS. 3 and 4, areutilized in the method of the present invention.

According to a preferred embodiment of the method of the presentinvention, magnetic domains 30 are oriented in the same direction suchthat they all have a first polarity, as described with respect to FIG.2A. Next, read head 70 scans the surface of the disk and variations inthe resistance of the sensor are recorded to identify defects. FIG. 5shows the change in sensor resistance for changes in magnetic fieldcorresponding to defects when the magnetic domains have a first magneticpolarity. When the sensor scans the disk, the magnetic field of the disksurface combines with that of the bias field to alter the magnetic fielddetected by the sensor, which is shown at point 110, i.e., a pointshifted away from the bias point 120. When the sensor scans across aspatial defect, the magnetic flux is decreased because the non-planarregion 65 is further away from the sensor than the disk surface 60 andtherefore contributes less to the magnetic field detected. Thus, themagnetic field 130 when the sensor is over a defect is moved towards thebias point 120. Accordingly, the sensor resistance is lower when thesensor is over a bad area than it is when the sensor is over a good area110. The resistance of the sensor is output as a signal discussed belowwith respect to FIG. 7.

FIG. 6 shows the sensor response, similar to that in FIG. 5, as itoccurs when the magnetic domains have the opposite polarity. After thefirst scan of the disk surface is complete, the magnetic domains 30 areoriented again such that they all have a second polarity that isopposite to the first. Thus, according to the method of the presentinvention, a second DC erase is then performed on the disk. The secondDC erase may be performed as described above. After orienting all of themagnetic domains to the second polarity, the next step of the methodinvolves performing a second scan of the disk surface with read head 70.In this second scan, because the polarity of the magnetic domains isreversed from its orientation during the first scan, the magnetic field110′ that is detected by the sensor (resulting from the combined fieldsof the magnetization of the disk surface and the bias field) is shiftedaway from the bias point 120 in the opposite direction from the shiftthat occurred in the first scan. As before, when the sensor scans over aspatial defect, the magnetic flux is decreased and the magnetic field130′ detected by the sensor is moved toward the bias point 120. However,because of the reversal of polarity, shifting toward the bias point inthis scan causes an increase in the sensor resistance rather than thedecrease seen with respect to FIG. 5.

It is to be understood that depending on the polarity used for the firstscan, the resulting signals may correspond to either FIG. 5 or FIG. 6;the signals related to the second scan (with the opposite polarity) willcorrespond to the other of the two figures.

Referring again to FIG. 4, it is noted that when the sensor scans over athermal defect it will increase in temperature, leading to an increasein resistance, regardless of the polarity of the magnetic domains.

As the sensor scans over the disk surface for each polarity, theresistance of the sensor is output to a signal processor (not shown).FIG. 7 shows the sensor resistance as a function of time as the sensorscans across the surface of the disk. In FIG. 7, the first signal 140corresponds to the situation described with respect to FIG. 5, where theresistance of the sensor increases over a spatial defect. This change inresistance is seen in region 145, where the signal increases rapidly andthen gradually declines. However, there is significant fluctuation inthe signal, including spike 170, which occurs as part of signal 140 nearthe end of region 145. Because of the variations in signal 140, it maybe difficult to determine whether signal fluctuations like spike 170should be interpreted as important signal information, or merely asnoise in the portion of the signal indicating the spatial defect (region145). According to the method of the present invention, however, asecond signal 150 is obtained, corresponding to the second scan of thedisk surface. In signal 150, a spatial defect is indicated by a decreasein the resistance of the sensor, shown in region 155. Although thissecond signal 150 also contains some fluctuation, spike 170 is seen inboth signals 140 and 150 as an increase in resistance. From thatinformation, spike 170 can be identified as indicating a thermal defect.Signal 160 is shown only for comparison; it is an output signal for amissing pulse scan, described above as a conventional method of testinga hard disk surface.

According to a further step of the preferred embodiment, signals 140 and150 are processed to improve the signal-to-noise ratios and enhance thedifferentiation of spatial and thermal defects. In order to identifyspatial defects, the signal processor subtracts signal 140 from signal150, which cancels out spike 170 and background noise, while retainingthe majority of the signals related to the spatial defect. As a result,the signal-to-noise ratio of the spatial defect signal is greatlyincreased over that of the conventional missing pulse test. In contrast,to identify thermal defects, the signal processor sums signal 140 andsignal 150. This summation causes the spatial defect information to besubstantially cancelled out, along with background noise, leaving thethermal defect information, spike 170, with a greatly improvedsignal-to-noise ratio.

The post-processing signals are shown in FIGS. 8A-8D, along with theconventional missing pulse signal and a comparison of the signal tonoise ratios. In FIGS. 8A and 8B, the spatial signal shows more than a20 dB improvement in signal to noise ratio over the missing pulsetechnique and the thermal signal also shows a significant improvement,though less than the spatial signal. As can be seen in FIG. 8C, evenwhen the missing pulse technique provides a good signal to noise ratio,the technique of the present invention still provides an improved ratio.Finally, FIG. 8D shows a 6 dB increase in signal to noise ratio for thethermal signal over the missing pulse technique where the defect has nospatial component (i.e., the defect is purely thermal, not spatial).

While the present invention has been particularly described with respectto the illustrated embodiments, it will be appreciated that variousalterations, modifications and adaptations may be made based on thepresent disclosure, and are intended to be within the scope of thepresent invention. While the invention has been described in connectionwith what are presently considered to be the most practical andpreferred embodiments, it is to be understood that the present inventionis not limited to the disclosed embodiments but, on the contrary, isintended to cover various modifications and equivalent arrangementsincluded within the scope of the appended claims.

1. In a disc drive, a system for distinguishing spatial and thermaldefects on perpendicular media comprising: a means for performing afirst DC erase having a first polarity on the perpendicular media; ameans for scanning the perpendicular media to produce a first signal; ameans for performing a second DC erase having a second polarity on theperpendicular media; a means for scanning the perpendicular media toproduce a second signal; a means for comparing the first signal with thesecond signal; a means for identifying locations on the perpendicularmedia having defects using one or both of said first and second signals;and, a means for using the comparison to distinguish the locationshaving thermal defects from the locations having spatial defects.
 2. Thesystem of claim 1, where the means for comparing adds the first andsecond signals.
 3. The system of claim 1, where the means for comparingsubtracts the first and second signals.
 4. The system of claim 1, wherethe means for comparing adds and subtracts the first and second signals.5. The system of claim 1, where the means for scanning the mediacomprise a read head.
 6. The system of claim 5, where the read headcomprises a GMR sensor.
 7. The system of claim 6, where the GMR sensorproduces the first and second signals in response to changes in amagnetic field.
 8. The system of claim 7, where changes in a magneticfield correspond to spatial defects.
 9. The system of claim 6, where theGMR sensor produces the first and second signals in response to changesin temperature.
 10. The system of claim 9, where changes in temperaturecorrespond to thermal defects.